1. Field of the Invention
The present invention relates to a process and an apparatus for performing analyses by liquid chromatography, wherein a syringe pump is used for injecting a fluid at high pressure into a separation column. More exactly, the invention relates to a process and apparatus to rectify the plunger stroke of said syringe pump in order to adjust the compressibility of the fluid it contains, whichever the operating pressure value is during an analysis in liquid chromatography.
The apparatus comprises a syringe pump actuated by a motor means and capable of drawing a preset quantity of solvent fluid from a container of same, then sending it, through a control valve, to a sample drawing zone and then to a column of chromatographic separation and finally to a detector.
2. Description of the Prior Art
As it is well known to those skilled in the art, the precision and reliability of liquid chromatography analysis are connected to several factors, the most important of which is the need of exactly controlling the quantity of solvent introduced into the column and consequently the constancy of the fluid flow to the column.
Up to now said delivery constancy was ensured by a motor means continuously acting on the piston pump, for example in the form of a step-by-step motor capable of ensuring a constant piston foward stroke and therefore, theoretically, a constant delivery feeding. However, it must be noticed that pressures involved during these analyses are very high, for example up to values of 500 kg/cm.sup.2, i.e. values at which the solvent compressibility, in particular that of certain fluids which are under supercritical temperature and pressure conditions as well as of certain solvents, cannot be disregarded. Amongst those fluids creating heavy problems of compressibility at said high values of pressure, the following can be considered: carbon dioxide, pentane, ammonia, when used under supercritical temperature and pressure conditions, and moreover alcohols, chlorinated, aromatic and aliphatic solvents. In these cases precision of the amount pumped can be lost, in that a constancy in the foward movement of the piston does not correspond to a constancy of delivered flow, because at high pressures said flow rate is reduced due to the solvent compressibility.